Add Firebase detection

[rviscomi]

cc @davideast

[fiws]

I don't think there is a header that identifies firebase but maybe just match all domains that end in web.app and firebaseapp.com?

[PierBover]

It seems pages served from Firebase have this header:

x-served-by: cache-lax8625-LAX

Or

x-served-by: cache-dfw18681-DFW

I'm not sure if this is exclusive to Firebase though.

[rviscomi]

Thanks @PierBover. Yeah it'd be good to know how many false positives/negatives there would be with that signal. Could there also be many variations of letters/numbers? That would be trickier to detect, but maybe we could extract a pattern.

[fiws]

I recently learned that firebase uses fastly for caching (found that out during the last fastly outage…). The x-served-by: cache […] header is set by fastly.

For me it has exactly the same value on my firebase page as on fastly.com (cache-muc13929-MUC).
So i think this header can not be used for detection.

[PierBover]

Thanks for the info @fiws .

So weird that Google uses Fastly instead of its own CDN.